Trimercapto-s-triazine, (referred to as "TMT-H.sub.3 " in the following description) as well as sodium salts of this trivalent acid, have already been described by A. W. Hofmann - Chem. Ber. 18 (1885), 2196 -2207. TMT-H.sub.3 was obtained by reacting 2,4,6-trichloro-s-triazine (cyanuric chloride) with sodium sulfide, followed by acidification. A monosodium salt of trimercapto-s-triazine was also isolated.
Nakamura et al. - Japan Kokai 49/1580 (Chem. Abstr. 81, 3972 b) prepared the monosodium salt of trimercapto-s-triazine (referred to as "TMT-Na" in the following description), in the form of the trihydrate.
According to the disclosure of Published German Patent Application DE-AS 22 40 549, heavy metals such as e.g. Cu, Cd, Ni, Hg, Ag, Pb can be separated as slightly soluble compounds from waste water using trimercapto-s-triazine or its water-soluble alkali-metal salts. TMT-Na is preferred in the exemplary comments of this document, the solubility in water of which is approximately 3% by weight; the di- and trisodium salt of trimercapto-s-triazine ("TMT-Na.sub.2 " and "TMT-Na.sub.3 ", respectively, in the following description) are also suggested as possible precipitation reagents. The saturation concentration of aqueous solutions of alkali-metal-, ammonium- and alkaline-earth metal salts of mono-, di- and trimercapto-s-triazines is indicated to be in the range 0.01 to 25% by weight at 25.degree. C. No information is provided in this document about the preparation of TMT-Na.sub.2 and TMT-Na.sub.3 and their characteristics and solubility in water.
Aqueous solutions of TMT-Na.sub.3 are commercially available and are used for separating heavy metal from the flue gas wash water of garbage incinerators, waste water in the mining industry and in galvanotechnical and chemical plants. The commercially available, aqueous TMT-NA.sub.3 solution has a concentration of 15% by weight (Publication TMT 15 of the Degussa company, 3/1986). The saturation concentration of TMT-Na.sub.3 in water is approximately 16% by weight at 0.degree. C. and approximately 25% by weight at 20.degree. C. Therefore, although the solubility of TMT-Na.sub.3 increases as the temperature rises - from approximately 0.78 mole TMT-Na.sub.3 per liter of H.sub.2 0 at 0.degree. C. to approximately 1.37 mole TMT-Na.sub.3 per liter of H.sub.2 0 at 20.degree. C. - only a solution of approximately 15% by weight has been practical in a commercially available pre-dissolved form. During storage at temperatures of about or even below 0.degree. C., TMT-Na.sub.3 crystallizes if the concentration is higher. Such a crystallization makes it considerably more difficult to handle the product. If heat-insulated or heated transport and storage containers were used, TMT-Na.sub.3 solutions with a higher concentration could be considered as a commercially supplied form of this product; however, this would make the product and its use more expensive.
TMT-Na.sub.3 has also been obtained as a crystalline nonahydrate by reacting cyanuric chloride with NaHS, Na.sub.2 S or a NaHS/Na.sub.2 S mixture in aqueous medium, adjusting the pH to values of preferably around 12.5 and crystallization - see Published German Patent Application DE-OS 37 29 029. In principle, aqueous solutions of TMT-Na.sub.3 can be prepared from crystalline TMT-Na.sub.3 nonahydrate at the time of use by dissolving in water; however, it is preferred to have a commercially available pre-dissolved form of TMT-Na.sub.3. For this reason, it would be desirable to be able to supply a solution of TMT-Na.sub.3 at a concentration of above 15% by weight. The actual active substance is the trimercapto-s-triazine, because it furnishes, together with the heavy metals, slightly soluble compounds which can be readily separated from waste water. Whether trimercapto-s-triazine is used as a solution containing TMT-Na or TMT-Na.sub.3 is of lesser importance, since the precipitation pH which is optimum for the recovery of particular metals is generally adjusted in the waste water.